In 3rd Generation Partnership Project (3GPP), an IP multimedia subsystem, specifically an IP Multimedia core network Subsystem (IMS), is defined in TS 23.228, TS 24.229 and the like for implementing multimedia services such as voice and SMS (Short Messaging Service) by using Session Initiation Protocol (SIP) at a mobile station or user equipment (UE). Also, in IR.92-IMS Profile for Voice and SMS in Global System for Mobile Communications Association (GSMA), requirements implement these services in LTE (Long Term Evolution) are specified, and in this document, voice and SMS services using the IMS implemented in the LTE are defined as VoLTE (Voice over LTE).
In the IMS, a packet switching (PS) domain can be used to provide services such as voice and SMS, which have been conventionally provided by using circuit switching (CS) domain. Since the services can be integrated into the PS domain facilities, it can be expected to reduce investment in the facilities. Also, the IMS can use broadband communication by QoS (Quality of Service) control to provide some merits to users, for example, provision of voice and video services of higher quality, provision of new services that cannot be achieved in the conventional CS domain such as presence, and the like.